This invention relates to a method of manufacturing a device employing a substrate of a material that exhibits the pyroelectric effect.
An important use of lithium niobate (LiNbO.sub.3) is in fabrication of electrooptic devices. In a typical electrooptic device, optical waveguides are formed in a monocrystalline substrate of LiNbO.sub.3 by diffusion of titanium into the crystal structure, and electrodes are deposited over a front surface of the substrate. Depending on the cut of the crystal, a buffer layer of SiO.sub.2 may be interposed between the substrate and the electrodes. Also, a buffer layer may be deposited on the back surface of the substrate. A potential difference is established between the electrodes, and the electric field that is thereby created in the LiNbO.sub.3 substrate influences the coupling of optical energy between the two waveguides.
A common use for an electrooptic device is as an optical switch. An optical switch may be used in an optical time domain reflectometer (OTDR) to direct light emitted from a light source into a fiber under test and direct reflected and back-scattered light from the fiber under test to a detector, depending on the field existing in the substrate. It is desirable that the field that affects the condition of the switch depend only on the potential difference between the electrodes.
LiNbO.sub.3 exhibits the pyroelectric effect. When a crystal of LiNbO.sub.3 undergoes a change in temperature, the pyroelectric effect causes a change in the spontaneous polarization of the material, and this produces a proportional electric field in the material along its Z-axis. Therefore, when the temperature of an optical switch based on LiNbO.sub.3 changes, the behavior of optical modes propagating through the waveguides is influenced not only by the potential difference established between the electrodes but also by the pyroelectric field.
The pyroelectric effect is discussed in C. H. Bulmer, W. K. Burns and S. C. Hiser "Pyroelectric Effects in LiNbO.sub.3 channel-waveguide devices", Appl. Phys. Lett., Vol. 48 (16), 1036 (1986), which confirms that the pyroelectric effect results in the performance of an electrooptic device based on LiNbO.sub.3 being highly dependent on temperature.
The pyroelectric effect is discussed further in P. Skeath, C. H. Bulmer, S. C. Hiser and W. K. Burns, "Novel electrostatic mechanism in the thermal instability of z-cut LiNbO.sub.3 interferometers", Appl. Phys. Lett., Vol. 49 (19), 1221 (1986), in which several possible methods to reduce the thermal instability of an electrooptic device are discussed. However, Skeath et al does not report on the efficacy of any of these methods.
The pyroelectric effect is self-extinguishing, since the pyroelectric field will result in charge being attracted to the Z faces of the substrate, and this charge will produce an electric field opposite in direction to the pyroelectric field. Given sufficient time (on the order of an hour in typical room air), an equilibrium state will be reached in which the accumulated surface charge produces a field that exactly cancels the pyroelectric field.
An integrated optic device, such as an optical switch, generally has conductive electrodes over some parts of its surface but not others. This inevitably causes the equilibrating process to proceed at a faster pace in the material directly underneath electrodes than in adjacent areas. There are two mechanisms for this. First, if the electrodes are connected to an external driver circuit, then a finite impedance will exist between them and "ground", where ground is any conductive material in the vicinity of the crystal (such as a metal package, or a block upon which the crystal sits). This finite impedance provides a path along which charge can move. Secondly, the sharp edges of the electrodes facilitate the ionization of surrounding air. As these mechanisms act, the surface charge density becomes different between regions with and without metal. Therefore, the electric field in the crystal will be different in the two regions. A plot of the field would show complicated fringing shapes at electrode edges. Waveguides located in various positions with respect to electrode edges will be subjected to various field strengths, so waveguide coupling will be disturbed as though a complex external field had been applied.
The equilibrating process can be made essentially instantaneous if the Z faces of the crystal are covered by conductive films and the films are electrically connected during the temperature change, since the conductive films contain mobile charge carriers that will respond immediately to the pyroelectric field by redistributing between the surfaces so as to cancel the pyroelectric field.
In I. Sawaki, H. Nakajima, M. Seino and K. Asama, "Thermally stabilized z-cut Ti: LiNbO.sub.3 waveguide switch", Proceedings, Optical Fiber Communications Conference, 1987, it is proposed that a semi-insulating film of indium tin oxide (ITO) be provided over the front surface of the substrate of an electrooptic device, covering the electrodes and the exposed surface of the substrate. The ITO film reduces the resistance between the electrodes of the device by at least three orders of magnitude and results in a more uniform distribution of pyroelectric surface charge over the device. A disadvantage of the structure disclosed by Sawaki et al arises from the difficulty of ensuring a continuous film of ITO over the front surface of the substrate, since the electrodes cause step coverage problems and the small spacing between the electrodes makes it difficult to deposit material between the electrodes.